The conventional process to produce fluids, such as oil and/or gas, from one or more subterranean zones is to drill a well into the zones. The zones are hydraulically fractured to increase the zone's permeabilities by providing fractures in the zones along which fluids can travel. The increased permeabilities increase the recovery of oil and/or gas by the well.
The process of hydraulically fracturing a target zone is composed of numerous steps. In the most common process, the steps include cementing a production casing in a well, loading an explosive device such as a perforating gun, lowering the device into the well by a wireline or similar device to the depth of the target zone, perforating the production casing in the well by triggering the explosive device, introducing a fluid into the target zone through the perforations to hydraulically fracture the target zone, and introducing a proppant into the fracture to restrict closure of the fracture after the fluid is removed from the well. The lengths of the fractures are typically limited to the target zone to prevent undesired fluids from other zones from flowing into the target zone along the fractures, to prevent the loss of the desired fluid into adjacent thief zones, and to prevent the commingled production of fluids from different zones. Thus, the casing is cemented in the wellbore not only for wellbore support but also to isolate the target zone from other zones. This technique is especially adapted for use in fracturing discrete, continuous zone-type deposits of the type shown in FIG. 1 from the well 50. The sandstone layer 54 a,b in such deposits is relatively thin (e.g., less than 200 feet) and is therefore easily targeted for fracturing by this technique.
The technique is not effective in recovering oil and/or gas from thick deposits, such as many tight-sands gas deposits. Gas contained in such deposits is much more difficult to recover than the gas in the continuous zone-type deposits exemplified in FIG. 1 due to their differing geologic characteristics distributed over great vertical heights. As shown in FIG. 2, the gas in tight sands deposits is contained in isolated, discontinuous sandstone stringers 58 of varying shapes and sizes which are in poor fluid communication with one another and are spaced over a vertical depth of typically more than about 500 feet and frequently over several thousand feet. Due to their highly heterogeneous nature, tight sands deposits include not one but a plurality of gas reservoir zones spaced over this large vertical depth interval. Due to the extreme thickness of tight sands deposits, the above-described conventional fracturing technique is of limited effectiveness in fracturing the numerous stringers 58 to permit the gas in the stringers to flow into the well 62. To fracture a multiplicity of such zones, the steps described above could be repeated for the larger stringers 58 and not the smaller stringers 58 due to cost prohibitions, thereby resulting in high well completion costs but also decreased oil and/or gas recoveries.
The fracturing technique described above is also not effective for fracturing zones located at greater depths than the bottom of the well. To employ the conventional fracturing technique, the well must be drilled to the depth of the target producing zone. This is often impractical and/or uneconomical for deep zones and/or for existing wells that for various reasons were originally drilled shallower than a desired target zone.
As a result of the high cost to drill and complete a well according to the above-noted technique, it is uneconomical to produce the oil and/or gas in many zones, especially zones located at depths below the bottom of the well or contained in the very thick tight sands deposits. Consequently, many oil and/or gas deposits are deemed uneconomic and therefore not recoverable.